This invention relates to surface coatings having radiative cooling properties and in particular to the use of such coatings on external surfaces of buildings to reduce the heat load to those buildings.
Radiative cooling refers to the process whereby a body will emit as radiation heat energy absorbed through normal convection and conduction processes.
The physics of black body radiation states that the wavelength at which a body will emit radiation is dependent on its temperature. For terrestrial temperatures, emission occurs in the infra-red (IR) wavelengths with a peak emission at approximately 11.4 μm. Incident solar radiation, on the other hand, corresponds to a black body temperature of 6000° K and is concentrated in the ultra-violet, visible and near IR wavelengths.
Not all of the radiation emitted by the Earth passes through to space. A significant portion of this radiation is absorbed in the Earth's atmosphere, particularly by the so-called “greenhouse gases” water vapour, carbon dioxide and ozone, and re-emitted back to the Earth's surface.
FIG. 1 shows atmospheric absorption as a function of wavelength. The species responsible for the various absorption peaks are identified on the horizontal axis. There is a low absorption “atmospheric window” in the region of 8-13 μm where the atmosphere is relatively transparent. A similar window exists for some wavelengths within the 1-5 μm band. Radiation from the Earth's surface within these wavelengths is likely to pass through these atmospheric windows to space rather than absorbed by the atmosphere and returned to the Earth's surface.
For the wavelengths having high atmospheric absorption there will be significant amounts of radiation in the atmosphere as that radiation is absorbed and re-emitted back to Earth. Conversely, for the wavelengths corresponding to these atmospheric windows there will be little radiation in the atmosphere as the majority of radiation emitted by the Earth at these wavelengths is allowed to pass through the atmosphere to space.
A “selective surface” is one that exploits the atmospheric window by preferentially emitting thermal energy at wavelengths corresponding to these atmospheric windows where there is reduced incident radiation which may be absorbed by the surface, that allow rapid transfer of that radiation to space and by being non-absorptive of radiation outside these wavelengths.